Systems and methods for production of artificial eumelanin

ABSTRACT

“Black” photoactive materials that comprise synthetic eumelanin polymers are provided, as are methods of making and using the polymers. The synthetic eumelanin polymers are made from the plant oil vanillin, and exhibit defined structural and chemical characteristics (e.g. homogeneity, solubility, etc.) that make them suitable for use in devices that require photoactive materials, such as solar cells.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/954,756 filed on Mar. 18, 2014, and incorporatessaid provisional application by reference into this document as if fullyset out at this point.

BACKGROUND OF THE INVENTION Field of the Invention

This invention generally relates to “black” photoactive materials thatcomprise synthetic eumelanin polymers. In particular, the inventionrelates to methods of making synthetic eumelanin polymers from vanillin,and their use in devices that require photoactive materials, such assolar cells.

Background

Our world faces an imminent global energy crisis that will be thedefining challenge for this generation of scientists and engineers. Thesearch for alternative fuels to power the future and alleviate humaneffects on the environment is a daunting task. Solar energy is foremostof these renewable energy sources due to its potential for providingnearly 274,000 terawatts of power per year (which exceeds the worldenergy consumption by a factor of about 17,000 times. Much progress hasbeen made in the development of inorganic based solar cells such asthose made from silicon. Since the invention of conductive polymers,scientists have been trying to provide replacements for inorganicphotovoltaic materials that are comprised of inexpensive plastics.Conductive plastics have the potential to create economical, processableand flexible alternatives to harvest energy from sunlight. The mostprominently studied active layer in polymer solar cells is aregioregular poly(3-hexylthiophene) (rr-P3HT) and[6,6]-phenyl-C61-butyric acid methylester (PCBM, derivative of C60) bulkheterojunction. Unfortunately, insufficient light absorption by rr-P3HTdue to its mismatch with the solar spectrum limits solar performance. Toimprove solar power conversion efficiencies of polymer solar cells,there is a critical need to design and synthesize novel conductingpolymers that absorb broadly and at the longer wavelengths that arerequired to better enable robust usage of the solar spectrum.

Melanins are a class of naturally occurring pigmentary macromoleculesfound in mammals. Compounds have a high degree of conjugation renderingthem a powerful radiation absorber with broadband photon absorptionspectrum that extends from the ultraviolet into the infrared range.Eumelanin, the black-brown variety of melanin that is responsible fordark-colored eyes, hair, and skin, acts as a natural photoprotector fromthe harmful radiation from the sun. Eumelanins are thought to be complexheterogeneous networks of randomly cross-linked biopolymers composed oftwo building blocks, 5,6-dihydroxyindole and5,6-dihydroxyindole-2-carboxylic acid, produced by oxidativepolymerization of the two monomers. Eumelanin is very resistant todamage caused by high temperature, chemical stresses, reactive oxygenspecies, ultraviolet radiation, X-rays, gamma rays, and alpha and betaparticles. It has extremely high absorption from 200 to 600 nm(extinction coefficient=2000-7600 cm⁻¹M⁻¹) and trails with a moderateabsorption up to 820 nm (500-800 cm⁻¹M⁻¹).

Interestingly, eumelanins are organic semiconductors. While the use ofeumelanins as organic semiconductors is an attractive proposition,unfortunately, natural eumelanin and eumelanin synthesized by existingprior art methods are extremely heterogeneous, insoluble in mostsolvents and do not have a well-defined structure. Thus elucidation ofstructure-property relationships is challenging or impossible. In theabsence of other forms of eumelanin, current research involvingelectronic devices based on eumelanin has only employed insolubleeumelanin pellets or thin, brittle films displaying very poormorphologies. Thus, it has not been possible to take advantage of thesemiconducting properties of eumelanins.

Heretofore, as is well known in the eumelanin synthesis arts, there hasbeen a need for an invention to address and solve the disadvantages ofprior art approaches. Accordingly it should now be recognized, as wasrecognized by the present inventors, that there exists, and has existedfor some time, a very real need for synthetic eumelanins and methods ofmaking and using synthetic eumelanins that would address and solve theabove-described and other problems.

Before proceeding to a description of the present invention, however, itshould be noted and remembered that the description of the inventionwhich follows, together with the accompanying drawings, should not beconstrued as limiting the invention to the examples (or embodiments)shown and described. This is so because those skilled in the art towhich the invention pertains will be able to devise other forms of thisinvention within the ambit of the appended claims.

SUMMARY OF THE INVENTION

The present disclosure provides synthetic eumelanins that arehomogeneous in compositions, soluble in a wide variety of solvents andwhich have a well-defined structure. Methods of making the syntheticeumelanins, a new core molecule that is used in the synthesis methods,and methods of using the synthetic eumelanins are also provided. In oneaspect, the starting material for the synthesis of the syntheticeumelanins is advantageously a plant oil, vanillin, extracted fromvanilla beans. Briefly, vanillin first passes through a series ofreactions to form the novel core compound 4,7-dibromoindole (methyl4,7-dibromo-5,6-dimethoxy-N-methyl-1H-indole-2-carboxylate), and the4,7-dibromoindole core molecule serves as a building block forproduction of well-defined, high performance conjugated polymers. Incontrast to thin film fabrication methods that depend on petroleum-basedpolymers, the present methods and compounds do not involve the use ofpetroleum starting materials/feedstocks and thus represent a sustainabletechnology. The synthetic eumelanin polymers are used, for example, as“black” photoactive materials for a variety of applications e.g. toproduce thin films used in solar cells, transistors, etc.

According to an embodiment, there is provided an indole as depicted inFormula I:

Where, R1, R2, R3 and R4 vary independently and may be the same ordifferent, and can be H, substituted or unsubstituted alkyl, substitutedor unsubstituted alkylester, substituted or unsubstituted alkoxy,substituted or unsubstituted aiyloxy moiety, substituted orunsubstituted polyethylene glycol (PEG), a macroinitator, a polymer or acrosslinker; A1 and A2 may be the same or different and are halogen,R₃Sn, B(OR)₂, OTf, SiR₃, or Si(OR)₃; and P is H or a protecting moiety.

According to another embodiment, there is taught herein a method ofmaking methyl 4,7-dibromo-5,6-dimethoxy-N-methyl-1H-indole-2-carboxylate(DBI), comprising reacting vanillin

to form DBI

According to still another embodiment, there is provided a polymer ofFormula III

where R1, R2, R3 and R4 vary independently and may be the same ordifferent, and can be H; substituted or unsubstituted alkyl; substitutedor unsubstituted alkylester; substituted or unsubstituted alkoxy;substituted or unsubstituted aryloxy; substituted or unsubstitutedpolyethylene glycol (PEG); a macroinitator; a polymer or a crosslinker;P is H or a protecting moiety; UCP is present or absent and if presentis an unsaturated carbon pair which is connected directly to a repeatunit of the polymer or is connected indirectly to a repeat unit of thepolymer through a substituted or unsubstituted aromatic; and n rangesfrom about 10 to about 1000.

According to a further embodiment, there is taught herein a method offorming a polymer comprising the step of reacting4,7-dibromo-5,6-dimethoxy-N-methyl-1H-indole-2-carboxylate (DBI)

with a compound of formula X1-UCP-X2, wherein UCP is an unsaturatedcarbon pair and X1 and X2 are the same or different and are chemicalgroups capable of displacing Br and forming a covalent bond at positions4 and 7 of said DBI.

Taught herein is an embodiment of a method of forming a polymercomprising the step of reacting4,7-dibromo-5,6-dimethoxy-N-methyl-1H-indole-2-carboxylate (DBI)

with a compound of formula

wherein UCP is an unsaturated carbon pair and wherein Y1, Y2, Y3 and Y4are the same or different and are selected from i) H or a saturated orunsaturated, branched or unbranched, substituted or unsubstituted alkylwith from about 1 to about 30 carbon atoms; or ii) OR where R is H or asaturated or unsaturated, branched or unbranched, substituted orunsubstituted alkyl with from about 1 to about 30 carbon atoms.

Additionally taught herein is a solution comprising a plurality ofpolymers of any of claims 6-13, and/or a plurality of unpolymerized orpartially polymerized repeat units thereof; and a solvent.

Further taught herein is a method of making a photoactive semi-conducivematerial comprising i) applying to a substrate, a solution comprising aplurality of polymers of any of claims 6-13, and/or a plurality ofunpolymerized and/or partially polymerized repeat units thereof; and asolvent; and ii) allowing said solvent to evaporate from said substrate,thereby forming a photoactive semi-conducive material on said substrate.

The foregoing has outlined in broad terms some of the more importantfeatures of the invention disclosed herein so that the detaileddescription that follows may be more clearly understood, and so that thecontribution of the instant inventors to the art may be betterappreciated. The instant invention is not to be limited in itsapplication to the details of the construction and to the arrangementsof the components set forth in the following description or illustratedin the drawings. Rather, the invention is capable of other embodimentsand of being practiced and carried out in various other ways notspecifically enumerated herein. Finally, it should be understood thatthe phraseology and terminology employed herein are for the purpose ofdescription and should not be regarded as limiting, unless thespecification specifically so limits the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to thedrawings in which:

FIG. 1. Synthesis of eumelanin-inspired core (DBI) 7 from vanillin(Scheme 1).

FIG. 2. Synthesis of compounds 9a and 9b (Scheme 2).

FIG. 3. The UV-vis absorption spectra of 7, 9a and 9b.

FIG. 4. Synthesis of the eumelanin-inspired polymer (Scheme 3).

FIG. 5. Absorption spectra (in solution) of exemplary small moleculesformed by functionalizing a DBI core.

FIG. 6. Emission spectra (in solution) of exemplary small moleculesformed by functionalizing a DBI core.

FIG. 7. HOMO and LUMO (highest occupied molecular orbital and lowestunoccupied molecular orbital) energy values of exemplary small moleculesformed by functionalizing a DBI core.

FIGS. 8A and B. A, generic arylene polymer; B, synthesis scheme for twoexemplary arylene polymers.

FIGS. 9A and B. Absorbance of the polymers in which R=dodecyl and2-ethylhexyl A, in solution and B, as a thin film. P1=dodecyl in THF;P2=2-ethylhelxy in THF; P3=2-ethylhelxy in DMF.

FIGS. 10A and B. A, synthesis scheme; B, absorbance spectrum for anexemplary poly(indolylene ethynylene) polymer.

FIG. 11 A-C. A, representative synthesis scheme for a poly(arylenevinylene) polymer in which R=dodecyl; B, absorbance data; C,fluorescence data for the polymer.

FIG. 12 depicts the solvatochromic properties of selected DBI-polymers.S: soluble, P: partly soluble and I: insoluble in a particular solvent.

FIG. 13. Exemplary alternative synthetic scheme for DBI.

FIG. 14A-D shows schematic representations of devices in which thesynthetic eumelanin polymers are used. A, polymer solar cell containingan active layer comprised of eumelanin-based donor polymers and a PCBMacceptor; B, an organic light-emitting diode containing an emissivelayer comprised of eumelanin polymers; C, an ion battery containingeumelanin polymers as electronic and ionic conductors; and D, afield-effect transistor containing eumelanin polymers as the chargetransport materials.

DETAILED DESCRIPTION

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings, and will herein be describedhereinafter in detail, some specific embodiments of the instantinvention. It should be understood, however, that the present disclosureis to be considered an exemplification of the principles of theinvention and is not intended to limit the invention to the specificembodiments so described.

The present disclosure provides novel well-defined eumelanin basedpolymers and synthesis routes for making the polymers. The polymers aremade using a novel indole molecule which serves as a core for thepolymer building blocks (subunits, repeat units, monomers), a genericrepresentation of which is presented in Formula I below:

In Formula I, R1, R2, R3 and R4 are variable groups which may be thesame or different, or two or three of which can be the same while thethird and/or fourth differs, and can be H, alkyl (e.g. C1 to C20branched or unbranched, substituted or unsubstituted alkyl, etc.), analkylester, an alkoxy moiety, an aryloxy moiety, polyethylene glycol(PEG), a macroinitator [e.g. poly(oxyethylene), poly(3-hexylthiophene),a α-methylstyrene-containing precopolymer, poly(alkoxyamine),poly(dimethylsiloxane), etc.], various a polymers or a crosslinker (e.g.bis(triethoxysilylpropyl)tetrasulfide, isocyanate-based crosslinkingagents, various acrylate and phenolic crosslinkers, etc.) A1 and A2 maybe the same or different and are halogens such as Br, I, F, Cl, etc. orother leaving groups such as OTf (triflate), SiOR₃, SnR₃, B(OR)₂, etc. Pis H or a protecting moiety, e.g. an atom or group of atoms that isnon-reactive or less reactive e.g. than the halogens at A1 and A2, andmay be, for example, alkyl (e.g. C1 to C20 branched or unbranched,substituted or unsubstituted alkyl, etc.), PEG, carbamates, acetamides,benzyl, or tosylamide.

In one aspect, the indole core molecule is a 4,7-dibromoindole, methyl4,7-dibromo-5,6-dimethoxy-N-methyl-1H-indole-2-carboxylate (DBI, FormulaII)

As noted above, exemplary variations on DBI can include alternativealkoxy moieties, aryloxy moieties at either or both the methoxys;alternative halogens or other leaving groups at either or both thebromine moieties; hydrogen or other alkyls at the nitrogen moiety;substitutions with an alkyl (e.g. PEG, etc.) at the unsubstituted carbonon the five-membered ring; and alkyl or alkylesters at the site of themethyl ester moiety.

The indole core molecules of the invention are advantageously made usingthe natural compound vanillin as the starting material.

As is evident from Formula I, the indole core molecule can be parasubstituted at the 4 and 7 positions by displacement of the leavinggroups A1 and A2. This property, plus varying of the R1, R2, R3 and R4groups and protecting group P, provides an avenue to making many diversepolymer subunits, as well as various copolymers and crosslinked polymernetworks of defined composition, thereby providing the opportunity totune polymer compositions to confer desired properties on the polymers.As a result, while the self-assembled synthetic eumelanin based polymersdescribed herein do mimic the heterogeneous network formed by naturaleumelanin, they also differ from and are superior to natural or priorart eumelanin in several significant ways. For example, the chemicalstructures of the new artificial polymers are well-defined. As describedabove, the polymers exhibit high tolerance to the introduction ofdiverse functional groups, making it possible to alter the optical andelectronic properties of the polymers for different applications. Forexample, the polymers in general are soluble in many common solvents,including water, but can also be designed to be more or less soluble, oreven insoluble, when necessary. Similar to natural eumelanin, these newpolymers absorb sunlight over a broad region (e.g. from about 400 toabout 800 nm), i.e. broader than that of a commonly used petroleum basedphotoactive polymer, rr-P3HT. However, this range can also be alteredfor particular applications. Other optical, electronic, chemical andphysical properties of the polymers can also be adjusted by decoratingthe polymers at one or more variable sites, e.g. by varying the R groupsof the DBI core material. The result is robust, processable syntheticeumelanin polymers with desired ranges of light absorption, chargemobility, film forming properties, reactivity or lack thereof,durability and longevity.

As used herein, “polymers” are molecules comprised of a plurality ofrepeat units (e.g. about 50 to 1000 or more repeat units). Polymers arethus typically of relatively high molecular weight, e.g. from at leastabout 5 to about 100 kDa or more in average molecular weight. Those ofskill in the art will recognize that the exact Mw will vary, dependingon the chemical composition of the components. Further, a product madewith polymers as described herein may comprise a variety of polymers ofdifferent lengths. Those of skill in the art will also understand thatpolymers typically become insoluble at an average Mw of between about100 kDa to about 150 kDa or higher, unless particular groups areintroduced into the polymer to increase solubility. For example, alkylchains may be introduced to promote solubility in hydrophobic solvents.The present polymers advantageously provide flexibility in compositionso that soluble high Mw polymers (e.g. from about 100 to about 150 kDaor even higher) can be made in soluble forms. In addition, the productsdescribed herein may also comprise oligomers or a mixture of polymersand oligomers. The term “oligomer” as used herein refers to molecules ofrelatively low molecular weight or partially polymerized repeat units(e.g. less than about 50, 25% or even fewer repeat units) than a“polymer”. Oligomers may comprise only a few repeat units (e.g. about 2to about 50).

Synthetic Eumelanins and Methods of Making Synthetic Eumelanins

Exemplary synthetic methods for making the polymer starting materialsand polymers described herein are presented in the Examples sectionbelow. In particular, Scheme 1 in FIG. 1, which is described in Example1, shows a synthesis scheme for DBI in which vanillin is the startingmaterial. According to the scheme, vanillin is in successive reactionsmethylated, nitrated, and brominated, a methoxy group is added to permitformation of a 5-membered ring, thereby forming an indole. Methylationof the ring nitrogen of the indole prevents further reaction at thatatom, and thus in the final product, DBI the two Br atoms generatereactivity at the 4,7 position on the indole ring. Several steps of themethod are or can be catalyzed by metals. An alternative method formaking DBI is presented in Example 4.

Example 1 and Example 2 also describe the synthesis of a variety ofnovel small molecules based on the exemplary indole core molecule DBI,although other indole core molecules of interest as described herein,may also be used as the basis for forming small molecules. In the caseof exemplary DBI, the small molecules are formed by replacing Br atpositions 4 and 7 with a variable group of interest, thereby forming adisubstituted indole. The variable groups each comprise an unsaturatedcarbon pair covalently bonded to a phenyl substituted with an (optional)variable R group. The unsaturated carbon pair reacts with Br atpositions 4 and 7 of DBI, displacing Br and forming a disubstitutedindole. This generic reaction is illustrated in Scheme A below, where“UCP” represents an unsaturated carbon pair and R represents the(optional) variable group attached to the phenyl moiety.

In such a reaction, the UCP may be a double or triple bond, and R═H; ora saturated or unsaturated, branched or unbranched, substituted orunsubstituted alkyl with from about 1 to about 30 total carbon atoms; analkoxy, a halogen (e.g. F, Cl, Br, etc.); an amine (NH₂, N-methyl,N-ethyl, N(CH₃)₂, N(CH₂CH₃)₂, etc); hydoxyl (OH); cyano (CN); nitro(NO₂); CF₃; SO₃H; CO₂H; ester, amides, etc. In some aspects, R is H,t-butyl, O-methyl, N(CH₃)₂, F, CN or NO₂. While Scheme A shows R para tothe UCP, it may also be ortho or meta. Further, the phenyl may besubstituted with more than one R in some applications.

Example 1 and Example 3 also describe modifications of the exemplaryindole core molecule DBI to form polymer repeat units (subunits), andpolymers made therefrom, by replacing Br of DBI with a variable group ofinterest that is capable of attaching to and linking two DBI coremolecules, thereby forming a polymeric chain of disubstituted indoles.However, those of skill in the art will recognize that other indole coremolecules described herein may also be used as the starting material. Inthe aspect in which DBI is used, the mechanism of joining two DBI's isdisplacement of Br by an unsaturated carbon pair (UCP), theunsubstituted carbon pair ultimately being attached directly to two (afirst and second) DBI core molecules. Scheme B generically illustratesthe generation of such repeat units:

In some embodiments, X1 and X2 may be the same or different and are, forexample and without limitation: R₃Sn, B(OR)₂, OTf, SiR₃, Si(OR)₃ etc. InScheme B, n ranges from about 10 to about 1000, without limitation.

In a second aspect, polymer repeat units are generated by reacting DBIwith a molecule that ultimately, in a polymer, is directly bonded to afirst indole core via a first UCP and is indirectly bonded to a secondindole core via an arylene moiety that is substituted with a second UCP.The second UCP bonds directly to the second indole core. The arylenemoiety may or may not be further substituted. In some aspects, theindole core is DBI, and this aspect is represented generically Scheme C:

“Arylene” refers to a substituent of an organic compound that is derivedfrom an aromatic hydrocarbon (arene) that has had a hydrogen atomremoved from two ring carbon atoms. In Scheme C, arylene may be, forexample and without limitation: unsubstituted or substituted aromaticssuch as benzenes, thiophenes, furan, pyrroles, pyridines, or polycyclicaromatic hydrocarbons. n ranges from about 10 to about 1000, withoutlimitation.

In one aspect of the invention, the arylene moiety is as depicted inScheme D:

In Scheme D, a copolymer is produced where the UCP is a double or triplebond and n ranges from about 10 to about 1000, without limitation. Thetwo UDPs are generally the same, but molecules with different UCPs arealso encompassed. Y1, Y2, Y3 and Y4 are variable groups whose presenceis optional (i.e. one more of Y1, Y2, Y3 and Y4 may be H). If present,they may be the same or different and may be, without limitation,independently selected from: a saturated or unsaturated, branched orunbranched, substituted or unsubstituted alkyl with from about 1 toabout 30 total carbon atoms; or an OR group where R is H or a saturatedor unsaturated, branched or unbranched, substituted or unsubstitutedalkyl with from about 1 to about 30 total carbon atoms; or CN, CF₃, orF. In some aspects, such as those depicted in FIGS. 8A and B, Y1 and Y4are the same and are OR, where R is dodecyl or 2-ethylhexyl. n rangesfrom about 10 to about 1000, without limitation.

In other aspects, what is provided is a polymer that does not includeunsubstituted carbon pairs as part of the linkage between repeat units.An exemplary polymer of this type is depicted in Formula IV:

In Formula IV, R1, R2, R3 and R4 are variable groups which may be thesame or different, or two or three of which can be the same while thethird and/or fourth differs, and can be H, alkyl (e.g. C2 to C20branched or unbranched, substituted or unsubstituted alkyl, etc.), analkylester, an alkoxy moiety, an aryloxy moiety, polyethylene glycol(PEG) a macroinitator [e.g. poly(oxyethylene), poly(3-hexyl thiophene),a α-methylstyrene-containing precopolymer, poly(alkoxyamine),poly(dimethylsiloxane), etc.], various a polymers or a crosslinker (e.g.bis(triethoxysilylpropyl)tetrasulfide, isocyanate-based crosslinkingagents, various acrylate and phenolic crosslinkers, etc.) In someaspects, R3 and R4 are RO, where R is H or a saturated or unsaturated,branched or unbranched, substituted or unsubstituted alkyl with fromabout 1 to about 30 total carbon atoms; PEG, a macroinitator, a polymeror a crosslinker (as above). P is H or a protecting moiety, for example,alkyl (e.g. C2 to C20 branched or unbranched, substituted orunsubstituted alkyl, etc.), PEG, carbamates, acetamides, benzyl, ortosylamide. n ranges from about 10 to about 1000, without limitation.Uses of Small Molecules Derived from DBI

Various small molecules with advantageous light absorbing and emittingproperties are described herein. Such small molecules are or can beused, for example and without limitation: as colorimetric sensors forvarious purposes, e.g. to measure pH, to detect the presence of varioussubstances such as metals in water (e.g. if the R groups are OH oranother suitable coordinating group); as additives (e.g. in polymerblends) for ultraviolet (e.g. sun) damage protection, either forpersonal use such as in sunscreens; or in paints or other products thatare applied to or coated onto structures (building, cars, etc.) orsubstances that are exposed to UV and/or sun; or as a component ofmaterials that are exposed to the UV and/or the sun (e.g. outdoorrecreational equipment, hoses, laboratory equipment, etc); or inwindows; in “plastic” protective sheeting; etc., in fibers used in UVprotective clothing; etc. These molecules can also serve as probes fordetection or imaging of biological analytes such as but not limited toviruses, DNA, and RNA, and recombinant forms thereof; etc.

Uses of the Synthetic Eumelanin Polymers

The synthetic eumelanin polymers disclosed herein are used in a widevariety of applications and devices. Generally, the application anddevices are any in which a photoactive material is required or desired,including but not limited to: solar cells; transistors and field effecttransistors (e.g. for radio-frequency identification (RFID) tags, forsecurity, anti-counterfeiting and logic devices, etc.); fuel cells;lighting (e.g. in plastic while lighting, displays e.g. screens fortelevision, computers, cell phones, watches, etc.; light emitting diodessuch as polymeric light emitting diodes (PLEDs) and organic lightemitting diodes (OLEDs); various sensors and biosensors; circuitry;thermoelectrics and organic electronics; pigment particles for a varietyof coating materials e.g. conductive coatings; metal remediation ofdrinking water (metal coordinating groups such as OH may be introduced);actuators; electrostatic shields; electromagnetic shields;

Solar Cells

According to one aspect of the invention, well-defined organic solubleeumelanin-based polymers are introduced as novel “black” photoactivedonor materials for incorporation in bulk heterojunction polymer solarcells, for example, monolithically linked or mechanically stacked tandemsolar cells. The proposed materials offer the advantage of harvestingmore solar radiation than prior art polymers and they can be decoratedwith functionalities for tuning the electronic and physical propertiessuch as charge mobility and film-forming properties. Enhanced solarpower conversion efficiencies are generated from mixtures of theseelectron donor assemblies which include the polymers and, for example,the electron acceptor, PCBM (a fullerene derivative[6,6]-phenyl-C61-butyric acid methyl ester).

Inks for Printing Photoactive Films and Photoactive Films Formed Thereby

The polymer building blocks and polymers described herein areadvantageously soluble in a variety of solvents and can use used to makeliquid compositions (inks) to form semiconductive ink compositions.Accordingly, provided herein are solutions comprised of building blocksof the polymers described herein and/or partially polymerized buildingblocks (oligomers) and/or fully polymerized polymers as describedherein, and a suitable solvent. The inks are suitable for use in formingfilms (e.g. thin films) by techniques known in the art, e.g. by spraytechnology resembling that of ink-jet printers and/or modificationsthereof such as those described in U.S. Pat. Nos. 8,597,973 and8,071,875 and in United States patent application 20140000700, theentire contents of each of which are hereby incorporated by reference.The technology generally involves applying e.g. by spraying or otherwisecoating the ink onto a substrate or support to form a film with desireddimensions, e.g. of a desired length, width and thickness.

Solvents which may be used to form the solutions described hereininclude, without limitation, dichloromethane, (DCM), tetrahydrofuran(THF), chloroform (CHCl₃), toluene, xylene, dimethylformamide (DMF),chlorobenzene, o-dichlorobenzene, and trichlorobenzene,dimethylsulfoxide (DMSO), methanol (MeOH) ethanol (EtOH) and water.

The solutions or inks that comprise the polymers, oligomers and orbuilding blocks thereof may also comprise other useful components, e.g.various colorants, dopants, metals or metals ions, other polymers,surfactants, and additives. Films formed in this manner are photoactiveand electrically conductive and suitable for use in variety devices,such as those described above (solar cells, transistors, etc.).

EXAMPLES Example 1. Eumelanin-Inspired Core Derived from Vanillin: A NewBuilding Block for Organic Semiconductors

An eumelanin-inspired core derived from the natural product, vanillin(vanilla bean extract) was utilized for the synthesis of eumelanininspired small molecules (4,7-disubstituted indoles) and polymers viaSonogashira cross coupling. The materials demonstrate that the methyl4,7-dibromo-5,6-dimethoxy-N-methyl-1H-indole-2-carboxylate core canserve as a new building block for organic semiconductors.

Organic semiconductors have attracted considerable attention due to thepromise of low cost, lightweight, and flexible large area electronicdevices.¹ Most of these materials are derived from petroleum-basedstarting materials. Due to the increasing demand on oil, it would beadvantageous to seek alternative sources for building blocks to developnew organic semiconductors.

In terms of chemical and functional diversity, nature is a great sourceof new building blocks for bioinspired organic semiconductors. One suchinspiration is the biopolymer, melanin which is a class of naturallyoccurring pigments found in the hair, eyes, skin, and the brain ofmammals and acts as a natural photoprotector against the harmful effectsof UV radiation.² Eumelanin is the black-brown variety of melanin andexist as a heterogeneous network, formed by the oxidative polymerizationof two monomers 5,6-dihydroxyindole (DHI) and5,6-dihydroxyindole-2-carboxylic acid (DHICA) (FIG. 1).³

Extensive research has been done on the optical, electronic, physical,metal chelating, and structural properties of natural and syntheticeumelanins.^(3,4) McGinness and Proctor's groundbreaking work onelectrical switching established eumelanins as amorphous organicsemiconductors.⁵ These eumelanins have excellent light absorptionranging from 200 nm to 700 nm in the electromagnetic spectrum,electrical conductivity reaching 10⁻⁵ S cm⁻¹ and exhibit good chargemobility as high as 2.1×10⁻³ cm² V⁻¹ s⁻¹.⁶ Hence, it seems appropriateand fitting to utilize the eumelanin indole moiety as a platform for thedevelopment of new organic semiconductors.^(2a)

Here, the synthesis of a eumelanin-inspired core molecule from thenatural product, vanillin, is presented. The eumelanin inspired buildingblock was designed so that functionalization on the 4,7-positions on thecentral indolic benzene ring can be feasible via transitionmetal-catalyzed cross-coupling reactions. Two new eumelanin-inspiredcompounds with interesting optoelectronic properties were synthesized bySonogashira cross-coupling.

The synthesis of the methyl4,7-dibromo-5,6-dimethoxy-Nmethyl-1H-indole-2-carboxylate (DBI)eumelanin-inspired core 7 is shown in Scheme 1 (depicted in FIG. 2).Vanillin (1) was methylated to give dimethoxybenzaldehyde (2), which wasnitrated to yield 3.⁷ Bromination of 3 using N-bromosuccinimide resultedin compound 4.⁸ A modified procedure was used to synthesize the olefin 5using methyl bromoacetate in aqueous sodium bicarbonate, followed bymicrowave-assisted Cadogan synthesis to afford 6.⁹ Finally, the indolewas N-methylated to avoid unwanted by-products during the cross couplingreactions. Compound 7 was synthesized with bromo groups at the 4 and 7positions of the indole moiety so it could serve as a universal partnerfor metal-catalyzed coupling reactions. This approach allowed for thefunctionalization at the 4 and 7 positions with alkynyl substituentsusing Sonogashira crosscoupling (Scheme 2, depicted in FIG. 3). Theethynyl group was chosen because of its ability to alter optoelectronicproperties by extended effective it-conjugation length.¹⁰

The coupling reaction was carried out using Pd(PPh₃)₄, CuI andtriethylamine (Et3N) as solvent and base to afford the products 9a and9b in high yields.¹¹ X-ray quality crystals of 9a were grown from avapour diffusion of ether and dichloromethane solutions (see ESI† forthe crystallographic data).

The UV-VIS absorption (FIG. 4) and photoluminescence (PL) of theeumelanin-inspired small molecules were investigated and summarized inthe Table 1. The 4,7-substituted eumelanin inspired small molecules 9aand 9b featured red-shifted absorbance maxima due to extendedconjugation of the system compared to the unsubstituted core 7 (absλ_(max) 308 nm). While the absorption band at lower wavelength (˜300 nm)corresponds to the indole core, the ethynyl substitution extends theconjugation through the eumelanin inspired core which resulted in theabsorption band ca. 400 nm. Moreover, the incorporation of the methoxygroup at the para position of the phenyl ring (9b) resulted in a slightred shift in absorption spectrum compared to 9a. The DBI core displayedvery weak fluorescence whereas 9a and 9b had PL maxima of 436 nm and 449nm, respectively. The PL quantum yields of the 9a and 9b in dilutechloroform solutions were 0.82 and 0.91, respectively. Optical bandgapswere estimated from the onset of the absorption are shown in Table 1.The compound 7 had a bandgap of 3.25 eV and as expected the4,7-substituted molecules 9a and 9b with extended conjugation showedreduced optical bandgaps of 2.94 and 2.87 eV, respectively.

TABLE 1 Optical and electrochemical properties of 7, 9a and 9b Ε_(opt)^(d) Ε_(ox) ^(e) Ε_(ox) ^(e) Ε_(ox-red) λ_(abs) ^(b) (nm) λ_(em) ^(b)(nm) Φ_(re) ^(c) (eV) (eV) (eV) (eV) 7 308^(a) — — −3.35 −5.76 −2.872.89 9a 384^(a), 209, 239 436 0.82 −2.94 −5.55 −2.70 2.85 9b 390^(a),353, 304, 241 449 0.91 −2.87 −5.45 −2.65 2.80 ^(a)λ_(max). ^(b)Measuredin dilute chloroform. ^(c)Quantum yields measured in dilute chloroformsolutions relative to quinine sulfate. ^(d)Measured from onset ofabsorption. ^(e)Calculated from onset of oxidation and reductionpotential.

Cyclic voltammetry measurements were carried out in dry degassedacetonitrile under inert atmosphere using 0.1 M tetrabutylammoniumhexafluorophosphate as the supporting electrolyte. A Ag/Ag⁺ referenceelectrode was calibrated against ferrocene/ferrocenium (Fc/Fc⁺) redoxcouple. The HOMO and LUMO (highest occupied molecular orbital and lowestunoccupied molecular orbital) energy values were calculated from theonset of the first oxidation and reduction potentials from the equationsE_(HOMO) (eV)=−[E_(ox) ^(onset)−(E_(1/2)(Fc/Fc⁺)+4.8] and E_(LUMO)(eV)=−[E_(red) ^(onset)−E_(1/2 (Fc/Fc) ⁺)+4.8], where E_(1/2) (Fc/Fc⁺)was the cell correction. The oxidation potential for DBI was −5.76 eVwhereas 9a and 9b values correspond to −5.55 eV and −5.45 eV,respectively. Both 9a and 9b had higher LUMO energy levels than the DBIcore (see Table 1). The calculated bandgap values were 2.89 eV, 2.85 eVand 2.80 eV for the 7, 9a and 9b, respectively, which indicated that theelectron donating methoxy group resulted in the lower bandgap. Thistrend was also observed for the estimated optical bandgaps. From thevoltammograms (S18), it was evident that the compounds had irreversiblereduction and oxidation potentials.

To further demonstrate the utility of the eumelanin inspired core 7 as abuilding block for organic semiconductors, a model conjugated polymerwas synthesized of via Sonogashira crosscoupling conditions. Thepolymerization of 7 and 1,4-bis(dodecyloxy)-2,5-diethynylbenzene (10),was selected because of the similarity of 10 to methoxy monomer 9b. Thisresulted in a red polymer with 36% yield (Scheme 3, shown in FIG. 5).The polymer was soluble in various solvents including THF, chloroform,toluene and chlorobenzene, and the structure was confirmed by 1^(H) NMRspectroscopy. Gel permeation chromatography showed a number averagemolecular weight (Mn) of 13.6 kDa, PDI=1.88. The photophysicalcharacteristics of the polymer, both in dilute solutions and thin films,were examined using UV-VIS absorption and fluorescence spectroscopy. Thepolymer exhibited an absorption maximum at 485 nm in solution and ared-shifted absorption maximum of 526 nm for polymer thin films. Greenfluorescence was observed for the polymer with an emission maximum at508 nm and the quantum yield in dilute chloroform was 0.60. Theelectrochemical properties of the polymer were investigated. The HOMOlevel for the polymer was similar to 9b (−5.47 eV); however, LUMO levelwas deepened to −3.44 eV. The morphology of the polymer thin film wascharacterized by AFM, as shown in the ESI.† The polymer thin filmappears to be composed of packed small grains varying in size and shapeaveraging 20 nm in diameter. Currently, work is continuing to optimizethe polymerization conditions in order to improve yield and obtainhigher molecular weight polymers.

In summary, this example shows that an eumelanin-inspired core moleculederived from vanillin can serve as a building block for the developmentof eumelanin-inspired organic semiconductors. Two new eumelanin-inspiredsmall molecules were synthesized in good yields. These materialsexhibited red-shifted absorption and emission compared to theeumelanin-inspired core DBI. This is attributed to the extendedconjugation due to the phenyleneethynylene linkage. Moreover, aneumelanin-inspired polymer was synthesized which showed promise foroptoelectronic devices. Current efforts focus on the synthesis andevaluation of optical and electronic properties of oligomers andpolymers based on the eumelanin-inspired core moieties.

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Example 2. Synthesis of Additional Small Molecules Using the DBI Core

Additional exemplary small molecules were formed by varying the R groupat positions 4 and 7 of DBI. In various syntheses, R was Br (parent DBImolecule), H, t-butyl, O-methyl, N(methyl)₂, F, CN and NO₂. Theabsorption spectra of the resulting small molecules are depicted in FIG.5, the emission spectra of selected compounds are presented in FIG. 6,and HOMO and LUMO values are presented in FIG. 7.

This example shows that the parent DBI core molecule can be successfullysubstituted at positions 4 and 7 with a wide variety of chemical groupswith differing properties, and the resulting small molecules eachdisplay useful absorption and emission properties. The small moleculesserve to demonstrate that the optical and electronic properties of DBIcan be tuned by simple substituting at the 4 and 7 positions on theindole ring in addition to the electron donating or withdrawing abilityof the R group at the para position the benzene ring. Moreover, some ofthese compounds exhibit a change in color at different pH values andupon metal binding and so can be employed in a variety of applicationsrelated to the measurement of pH and the detection of metals.

Example 3. Design and Synthesis of Well-Defined, SolubleEumelanin-Inspired Polymers

The DBI core molecule was successfully used to synthesize a varietypolymer building blocks and polymers with useful photovoltaicproperties.

Arylene Polymers:

FIG. 8A depicts a generic arylene polymer subunit and Figure B depictsan representative synthesis scheme in which variable R groups (e.g.dodecyl and 2-ethylhexyl) are incorporated into the polymers. Theresulting polymers were soluble in CHCl₃, tetrahydrofuran (THF), tolueneand chlorobenzene Table 2 shows the solubility data obtained for THF.

TABLE 2 Solubility of arylene polymers in CHCl₃ and THF R = Dodecyl R -Ethylhexyl R - Ethylhexyl (THF) (THF) (DMF) Mn* (kDa) 13.6 8.9 5.8 Mw**25.5 17.6 10.0 (kDa) PDI*** 1.88 1.97 1.92 Yield (%) 36 17 13 *Numberaverage molecular weight measured by gel permeation chromatography:**Number average molecular weight measured by gel permeationchromatography: ***Polydispersity index

FIG. 9 shows absorbance of the polymers in solution (A) and as a thinfilm (B), and Table 3 provides related data.

TABLE 3 Absorbance parameters P2 P3 P1 R = ethylhexyl R = ethylhexyl R =dodecyl (THF) (THF) λ_(max) (solution) 485 nm 462 nm 454 nm λ_(max)(film) 485 nm 507 nm 465 nm λ_(ems) (solution) 485 nm 505 nm 503 nm Eg(eV) 2.41 2.42 2.43Poly(Indolylene Ethynylene) Polymers

An exemplary poly(indolylene ethynylene) polymer was made using DBI asthe core molecule. The resulting polymer was soluble in common solventssuch as CHCl₃ and dichloromethane (DCM) and exhibited good film-formingproperties. The synthesis scheme is shown in FIG. 10A and absorbancedata is presented in FIG. 10B. Tables 4 and 5 present physical andabsorbance characteristics of the polymer, respectively.

TABLE 4 Physical characteristics of a poly(indolylene ethylnylene)polymer Mn (kDa) 4.0 Mw (kDa) 8.5 PDI 1.88 Yield (%) 36

TABLE 5 Absorbance characteristics of a poly(indolylene ethylnylene)polymer λ_(max) (solution) 475 nm λ_(max) (film) 491 nm λ_(ems)(solution) 525 nmPoly(Arylene Vinylene) Polymers

Poly(arylene vinylene) polymers were made using DBI as the coremolecule. The resulting polymers were soluble in common solvents such asCHCl₃, THF, DCM and toluene, and exhibited good film-forming properties.A representative synthesis scheme for a polymer in which R=dodecyl isshown in FIG. 11A and absorbance and fluorescence data is presented inFIGS. 11B and C, respectively. Tables 6 and 7 present physical andabsorbance characteristics, respectively, of a poly(arylene vinylene)polymer in which R=dodecyl

TABLE 6 Physical characteristics of a poly(arylene vinylene) polymer inwhich R = dodecyl in CHCl₃ solvent Mn (kDa) 413 Mw (kDa) 26 PDI 2.0Yield (%) 39

TABLE 7 Absorbance characteristics of a poly(indolylene ethylnylene)polymer λ_(max) (solution) 435 nm λ_(ems) (solution) 547 nm

FIG. 12 depicts the solvatochromic properties of selected DBI-polymers.As can be seen, the polymers have diverse chromic (optical) propertiesin different solvents mainly due the polarity of the solvent, animportant factor when fabricating optical devices.

In summary, this Example shows that a wide variety of photoactivepolymers with semiconductor properties can be made in good yield and ina relatively facile manner, starting from the core structure DBI, whichin turn is synthesized beginning with the plant oil vanillin.

Example 4. Alternative DBI Synthesis Method

In some aspects, DBI is synthesized from the natural product, vanillin(e.g. as extracted from vanilla beans). However, those of skill in theart will recognize that other alternative synthetic schemes may beemployed to arrive at this same novel molecule. An exemplary alternativesynthetic scheme is presented in FIG. 13. As can be seen, in thisalternative scheme, the starting material that was used is6-nitroveratraldhyde which is an intermediate in the vanilla synthesisroute. This Example shows that DBI can be synthesized by alternativeroutes that do not use vanillin as a starting material.

Example 5. “Black” Photoactive Materials for Organic Solar Cells:Eumelanin-Based Polymers

Eumelanin-based synthetic polymers described herein are new lightabsorbing donors which absorb sunlight over a broad region (400-800 run)similar to the natural biopolymer, and broader than that of rr-P3HT.Given the synthetic eumelanin core structure, the opportunity todecorate the polymer at multiple sites is available via the R positionsof the DBI starting material, which can be exploited to yield robust,processable polymers with tunable light absorption, durability andlongevity.

Accordingly, new, highly light-absorbing active layers for enhancedsolar power conversion efficiencies are generated from a mixture ofelectron donor assemblies comprising the polymers described herein andthe electron acceptor, PCBM (the fullerene derivative[6,6]-phenyl-C61-butyric acid methyl ester). A schematic illustration ofan organic solar cell utilizing these components is shown in FIG. 14.Accordingly, also provided herein are compositions comprising one ormore (e.g. at least one) polymer as described herein (or repeat unit oroligomer thereof) and graphene or graphene oxide, generally in the formof, for example, a fullerene or a carbon nanotube. Exemplary fullerenesinclude but are not limited to: [6,6]-phenyl-C61-butyric acidmethylester (PC₆₁BM), [6,6]-phenyl-C71-butyric acid methylester (PC₇₁BM), indene-C₆₀ (IC₆₀BA), and indene-C₇₀ (IC₇₀BA).

Additional details of the invention may be found in one or moreappendices attached hereto, the disclosure(s) of which are incorporatedherein by reference as if fully set out at this point.

The term “at least” followed by a number is used herein to denote thestart of a range beginning with that number (which may be a rangerhaving an upper limit or no upper limit, depending on the variable beingdefined). For example, “at least 1” means 1 or more than 1. The term “atmost” followed by a number is used herein to denote the end of a rangeending with that number (which may be a range having 1 or 0 as its lowerlimit, or a range having no lower limit, depending upon the variablebeing defined). For example, “at most 4” means 4 or less than 4, and “atmost 40%” means 40% or less than 40%. Terms of approximation (e. g.,“about”, “substantially”, “approximately”, etc.) should be interpretedaccording to their ordinary and customary meanings as used in theassociated art unless indicated otherwise. Absent a specific definitionand absent ordinary and customary usage in the associated art, suchterms should be interpreted to be ±10% of the base value.

When, in this document, a range is given as “(a first number) to (asecond number)” or “(a first number)−(a second number)”, this means arange whose lower limit is the first number and whose upper limit is thesecond number. For example, 25 to 100 should be interpreted to mean arange whose lower limit is 25 and whose upper limit is 100.Additionally, it should be noted that where a range is given, everypossible subrange or interval within that range is also specificallyintended unless the context indicates to the contrary. For example, ifthe specification indicates a range of 25 to 100 such range is alsointended to include subranges such as 26-100, 27-100, etc., 25-99,25-98, etc., as well as any other possible combination of lower andupper values within the stated range, e.g., 33-47, 60-97, 41-45, 28-96,etc. Note that integer range values have been used in this paragraph forpurposes of illustration only and decimal and fractional values (e. g.,46.7-91.3) should also be understood to be intended as possible subrangeendpoints unless specifically excluded.

It should be noted that where reference is made herein to a methodcomprising two or more defined steps, the defined steps can be carriedout in any order or simultaneously (except where context excludes thatpossibility), and the method can also include one or more other stepswhich are carried out before any of the defined steps, between two ofthe defined steps, or after all of the defined steps (except wherecontext excludes that possibility).

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings, and is herein described indetail, some specific embodiments. It should be understood, however,that the present disclosure is to be considered an exemplification ofthe principles of the invention and is not intended to limit it to thespecific embodiments or algorithms so described. Those of ordinary skillin the art will be able to make various changes and furthermodifications, apart from those shown or suggested herein, withoutdeparting from the spirit of the inventive concept, the scope of whichis to be determined by the following claims.

Further, it should be noted that terms of approximation (e. g., “about”,“substantially”, “approximately”, etc.) are to be interpreted accordingto their ordinary and customary meanings as used in the associated artunless indicated otherwise herein. Absent a specific definition withinthis disclosure, and absent ordinary and customary usage in theassociated art, such terms should be interpreted to be plus or minus 10%of the base value.

Thus, the present invention is well adapted to carry out the objects andattain the ends and advantages mentioned above as well as those inherenttherein. While presently preferred embodiments have been described forpurposes of this disclosure, numerous changes and modifications will beapparent to those skilled in the art. Such changes and modifications areencompassed within the spirit of this invention as defined by theappended claims.

The invention claimed is:
 1. A polymer of Formula III

where R1, R2, R3 and R4 vary independently and may be the same ordifferent, and can be H; substituted or unsubstituted alkyl; substitutedor unsubstituted alkylester; substituted or unsubstituted alkoxy;substituted or unsubstituted aryloxy; substituted or unsubstitutedpolyethylene glycol (PEG); a macroinitator; a polymer or a crosslinker;P is H or a protecting moiety; UCP is present or absent and if presentis an unsaturated carbon pair which is connected directly to a repeatunit of the polymer or is connected indirectly to a repeat unit of thepolymer through a substituted or unsubstituted aromatic; and n rangesfrom about 10 to about
 1000. 2. The polymer of claim 1, wherein saidprotecting moiety is methyl, ethyl, PEG, a carbamate, an acetamides,benzyl, or tosylamide.
 3. The polymer of claim 1, wherein one or more ofR1-R4 is crosslinked to a second polymer of Formula III.
 4. The polymerof claim 1, wherein UCP is present and said polymer has a formula


5. The polymer of claim 4, wherein said UCP is a triple bond which isconnected directly to said repeat unit of the polymer and the polymer is


6. The polymer of claim 4, wherein said UCP is connected indirectly tosaid repeat unit of the polymer and the polymer is

where Y1, Y2, Y3 and Y4 are the same or different and are selected fromi) H or a saturated or unsaturated, branched or unbranched, substitutedor unsubstituted alkyl with from about 1 to about 30 carbon atoms; orii) OR where R is H or a saturated or unsaturated, branched orunbranched, substituted or unsubstituted alkyl with from about 1 toabout 30 carbon atoms.
 7. The polymer of claim 6, wherein said UCP is adouble bond, Y1 and Y 4 are H and Y2 and Y3 are RO, where R is dodecyl.8. The polymer of claim 6, wherein said UCP is a triple bond, Y1 and Y 4are H and Y2 and Y3 are RO, where R is dodecyl or 2-ethylhexyl.
 9. Thepolymer of claim 8, wherein said polymer is: